Automatic furnaces of this type are suitable, for instance, to control the start-up process of a burner which produces heat in a heating system. Gas is used as the fuel in such a system.
An automatic furnace of this type is known from U.S. Pat. No. 4,099,906, in which the ignition element used for the gas/air mixture is an electric igniter known as a "hot surface igniter". Such electric igniters have been used for years in countries with a 120 Volt a.c. power network and are obtainable on the market. However, the production of such electric igniters for 230 Volt operation is prevented by the properties of the materials, e.g., silicon carbide, used in such electric igniters. For this reason such electric igniters are not yet available.
Several possibilities exist for the utilization of a 120 volt igniter in a 230 Volt network. First, there is the possibility of using a transformer in order to convert the 230 V voltage to 120 V. In view of the necessary power output of approximately 400 Watt, such a transformer would be of such a size that it could not fit into a conventional housing of an automatic furnace. Because of the space availability on a burner, the automatic furnace housing cannot be enlarged. Furthermore, such a transformer would represent a considerable cost factor.
The utilization of phase control by means of thyristors offers an alternative solution. This solution is also disadvantageous in that it necessitates considerable expense for interference suppression means in order to attenuate the radio interferences, which invariably occur in such circuits, to a level in conformity with legal provisions. In view of the high performance necessary, these interference suppression means also take up much space.